Method for producing plants with suppressed photorespiration and improved CO2 fixation

ABSTRACT

The invention relates to a method for the production of plants with suppressed photo-respiration and improved CO 2  fixation. In particular, the invention relates to a re-use of phosphoglycolate produced in photorespiration. The reaction product will be converted to a component that may be reintegrated into the plant assimilatory metabolism inside the chloroplast. This is accomplished by the transfer of genes derived from glycolate-utilizing pathways from bacteria, algae, plants and/or animals including humans into the plant nuclear and/or plastidial genome. The method of the invention leads to a reduction of photorespiration in C 3  plants and by this will be of great benefit for food production especially but not exclusively under non-favourable growth conditions.

RELATED APPLICATIONS

The present application claims priority to International Application No. PCT/EP03/05398, filed on May 23, 2003, which claims priority to European Application No. 02011578.8 filed on May 27, 2002, each of which is incoporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention is directed to plants exhibiting suppressed photorespiration and improved CO₂ fixation and methods for producing such plants.

BACKGROUND OF THE INVENTION

Many efforts have recently been made to improve growth and resistance of crop plants. Some of the most important crop plants, e.g. rice, wheat, barley, potato, belong to the so-called C₃ plants. Only a few important crop plants, like corn and sugar cane, are C₄ plants. CO₂ fixation in C₃ plants is primarily catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase (RUBISCO) which is located in the chloroplasts. The enzyme RUBISCO catalyzes two reactions: carboxylation and oxygenation of ribulose-1,5-bisphosphate. The product of the first reaction are two molecules of 3-phosphoglycerate which enter the Calvin cycle to form starch and ribulose-1,5-bisphosphate. The products of the oxygenase reaction are one molecule each of 3-phosphoglycerate and phosphoglycolate (Goodwin and Mercer, 1983). The latter is converted to 3-phosphoglycerate in a biosynthetic pathway known as photorespiration (see FIG. 1). In the course of this complex sequence of reactions, one molecule of CO₂ is released and lost for the plant. This loss of CO₂ reduces the formation of sugars and polysaccharides in the plant and thus reduces their productivity. Furthermore, NH₃ is released which has to be refixed. These effects are exacerbated further when plants are grown under suboptimal water supply. Here, leaf stomata are closed and the intercellular oxygen concentration rises because of molecular oxygen released from the light reactions of photosynthesis. High amounts of phosphoglycolate are produced that enter the photorespiratory cycle. It has been estimated that plants lose approximately 25% of the already fixed carbon due to photorespiration. This cycle is absolutely intrinsic to all C₃ plants because of the oxygenase activity of RUBISCO (Leegood et al., 1995; Tolbert, 1997).

The importance of photorespiration for plant growth and yield has been shown by several experiments where the atmospheric CO₂ concentration has been artificially raised in greenhouse experiments. Significant increases in the performance of several crop species have been observed already when the CO₂ concentration is doubled (Kimball, 1983; Arp et al., 1998). However, this approach is not applicable to the large, open areas used for agricultural production.

C₄ plants have evolved a mechanism to avoid these losses. They have employed enzymes already present in their C₃ ancestors, but changed the degree of expression as well as the localisation on a subcellular and cell-type specific level. By separating primary and secondary carbon fixation in two different tissues, they drastically increase the local CO₂ concentration at the site of RUBISCO activity. Shortly, the first CO₂ fixation takes place in the cytoplasm of mesophyll cells and is catalyzed by PEPC, an enzyme without intrinsic oxygenase activity and significantly higher affinity to its substrate compared to RUBISCO. The resulting C₄ acid diffuses into the gas tight bundle sheath and is here decarboxylated to liberate CO₂. The remaining monocarbonic acid serves to regenerate the primary CO₂ acceptor in the mesophyll. This CO₂ concentration mechanism results in a complete suppression of photorespiration and an oxygen-insensitive photosynthesis (Kanai and Edwards, 1999). A similar mechanism with a temporal, instead of spatial, separation of enzymatic activities is applied by the crassulacean acid metabolism (CAM) plants (Cushman and Bohnert, 1999).

Beside these mechanisms depending on the cooperation of two different cell types some aquatic plants have developed C₄-like mechanisms working within one cell. Here, primary and secondary CO₂ fixation take place in one cell, but in different compartments. Whereas PEPC activity is restricted to the cytoplasm, CO₂ release and refixation similar to C₄ plants take place in the chloroplast. This unicellular C₄-like pathway results in a partial suppression of photorespiration with reduced sensitivity to oxygen (Reiskind et al., 1997).

Several attempts have been described to transfer C₄- or C₄-like pathways or components of this pathway to C₃ plants. Mostly, overexpression of PEPC has been used so far. Three groups applied expression of the maize PEPC cDNA or gene under control of different promoters (Hudspeth et al., 1992; Kogami et al., 1994; Ku et al., 1999). Although increases in PEPC activity levels up to 100-fold were detected with the complete intact maize gene in rice (Ku et al., 1999), there were only weak impacts on plant physiology and growth performance (Matsuoka et al., 2001, see also EP-A 0 874 056). Recently, the overexpression of PEPC and malate dehydrogenase from Sorghum in potato has been described, but in this case expression levels were low and no modification of photosynthetic parameters could be observed (Beaujean et al., 2001). PEPC cDNAs from bacterial source have been overexpressed in potato by Gehlen et al. (1996) with some minor impact on photosynthetic parameters. The combination of this enzyme with the additional overexpression of a NADP⁺-malic enzyme (ME) from Flaveria pringlei targeted to the chloroplast enhanced these effects without any impact on plant growth or yield (Lipka et al., 1999). For rice, it has been recently described that the overexpression of a phosphoenolpyruvate carboxykinase (PCK) from Urochloa panicoides targeted to the chloroplast results in the induction of endogenous PEPC and the establishment of a C₄-like cycle within a single cell. However, no enhanced growth parameters were observed (Suzuki et al., 2000; see also WO 98/35030).

Therefore, despite several attempts to improve CO₂ fixation and reduce photorespiration, until now no method has been provided that leads to an improvement of growth, productivity, and/or yield for agricultural crop plants. All these attempts were aiming to concentrate CO₂ at the site of fixation in order to suppress the oxygenase activity of RUBISCO.

Many bacteria have evolved biochemical pathways to metabolize glycolate, the primary product of the oxygenase activity of RUBISCO. For Escherichia coli, this pathway has been described in great detail (Lord, 1972; Pellicer et al., 1996). E. coli is capable of growing on glycolate as the sole carbon source. As summarised in FIG. 4, glycolate is first oxidized to glyoxylate. This reaction is brought about by a multiprotein complex that is capable of oxidizing glycolate in an oxygen-independent manner. The proteins necessary for glycolate oxidation in E. coli have been analysed and it has been shown that the open reading frames D, E, and F of the glycolate oxidase operon glc are encoding the components of the active enzyme. In the next reaction step, two molecules of glyoxylate are ligated by glyoxylate carboligase (GCL) to form tartronic semialdehyde (TS) and CO₂ is released in this reaction. TS is further converted to glycerate by TS reductase (TSR). Glycerate is integrated into the bacterial basal carbon metabolism.

A similar pathway is also applied as a photorespiratory cycle in some green algae and cyanobacteria (Nelson and Tolbert, 1970; Ramazanov and Cardenas, 1992). In this case, glycolate oxidation is seemingly catalysed by glycolate dehydrogenase located inside the mitochondria. Again, this enzyme is not oxygen-dependent and uses organic electron acceptors like Nicotin-Adenosin-Dinucleotid (NAD⁺) instead. The further metabolism of glyoxylate seems to be similar to the pathway as described for E. coli.

SUMMARY OF THE INVENTION

The present invention relates to methods for suppressing photorespiration and increasing productivity and/or yield in crop plants. Accordingly, the present invention provides methods for producing plants with suppressed photorespiration and improved CO₂ fixation. In particular, the invention relates to a re-use of phosphoglycolate produced in photorespiration. The reaction product is converted to a component that may be reintegrated into the plant assimilatory metabolism inside the chloroplast. This is accomplished by the transfer of genes derived from glycolate-utilizing pathways from bacteria, algae, plants and/or animals including humans into the plant nuclear and/or plastidial genome. The methods of the invention lead to reduced photorespiration in C₃ plants and, therefore, are of great benefit for food production, especially but not exclusively under non-favourable growth conditions.

In one nonlimiting embodiment, the present invention provides a method for the production of plants with suppressed photorespiration and improved CO₂ fixation, such that the method includes introducing one or more nucleic acids into a plant cell, plant tissue or plant, wherein the introduction of the nucleic acid(s) results in a de novo expression of polypeptides having the enzymatic activities of (i) glycolate oxidase or glycolate dehydrogenase, (ii) glyoxylate carboligase and (iii) tartronic semialdehyde reductase.

BRIEF EXPLANATION OF THE FIGURES

FIG. 1: The photorespiratory pathway

FIG. 2: The C₄-pathway

FIG. 3: The C₄-like unicellular pathway

FIG. 4: The bacterial glycolate-utilizing pathway

FIG. 5: A novel plastidial pathway in accordance with the present invention

FIG. 6: Schematic representation of exemplary T-DNA constructs useful for plant transformation

FIG. 7: Schematic representation of exemplary vector constructs useful for plant transformation

FIG. 8: A construct for the polycistronic plastidic expression of the components of the pathway

FIG. 9: Glycolate dehydrogenase activity of an Arabidopsis thaliana homologue to the E. coli glc operon.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods for the production of plants with suppressed photorespiration and improved CO₂ fixation, the methods comprising introducing into a plant cell, plant tissue or plant one or more nucleic acids, wherein the introduction of the nucleic acid(s) results in a de novo expression of polypeptides having the enzymatic activities of (i) glycolate oxidase or glycolate dehydrogenase, (ii) glyoxylate carboligase and (iii) tartronic semialdehyde reductase.

In particular, the invention relates to a re-use of the phosphoglycolate produced in photorespiration. The reaction product will be converted to a component that may be reintegrated into the plant assimilatory metabolism inside the chloroplast. This will be accomplished by the transfer of genes derived from glycolate-utilizing pathways from bacteria, algae, plants and/or animals including humans into the plant nuclear and/or plastidial genome. The present invention provides a reduction of photorespiration in C₃ plants and by this is of great benefit for food production, especially but not exclusively under non-favourable growth conditions.

The present invention provides methods to install or introduce a glycolate oxidizing activity inside the chloroplast. For this, several enzymes have to be transferred to the chloroplast. This can be brought about, for example, by nuclear transformation of plant cells, plant tissue or plants with the coding sequence of the respective protein fused to a chloroplast transit peptide or by direct transformation of the chloroplast genome. Promoters controlling the expression of the respective transgenes may be, for example, constitutive or controlled by environmental or technical means. The enzymes may be transferred to plant cells, plant tissue or plants, for example, on single plasmid constructs or independently on several constructs. General techniques for gene integration are well known in the art and include Agrobacterium-mediated transfer, electroporation, microinjection, or chemical treatment. The present invention is applicable to any plant, plant cells (including, for example, algal cells), or plant tissues. In one non-limiting embodiment, the plant is a C₃ plant. In other non-limiting embodiments, the plant is a potato or tobacco plant.

A pathway in accordance with the present invention is shown in exemplary FIG. 5: The first protein in the new biochemical pathway is an enzyme catalysing the oxidation of glycolate in an oxygen-independent manner with organic cofactors. Oxidases useful for this purpose include, for example, bacterial glycolate oxidases and algal glycolate dehydrogenases as well as mammal glyoxylate reducing activities and functional homologues derived from plants.

TABLE 1 Glycolate-oxidising enzymes Enzyme Activity Glycolate oxidase oxygen dependent (GO; EC 1.1.3.15) peroxisomal plant photorespiration Glyoxylate oxidoreductase NAD/NADP dependent (GOR; EC 1.1.1.26/79) cloned from human source equilibrium unclear Glycolate dehydrogenase NAD dependent = (GDH; EC 1.1.99.14) E. coli glycolate oxidase activity described for algal mitochondria

The protein may be constituted, for example, by one or multiple polypeptides. Importantly, the use of enzymes from these sources will prevent the formation of reactive oxygen species that are produced by the higher plant glycolate oxidases localized inside the peroxisomes.

In preferred non-limiting embodiments, polypeptides having the enzymatic activity of a glycolate oxidase are those encoded by the E. coli glc operon. Preferred non-limiting embodiments use polypeptides which comprise the amino acid sequences of SEQ ID NOS: 2 (Glc D), 4 (Glc E) and 6 (Glc F). Therefore, nucleic acids comprising a polynucleotide sequence of SEQ ID NOS: 1, 3 and 5 can be used in accordance with the present invention.

In another non-limiting embodiment, human glyoxylate reductase having the activity of a glycolate dehydrogenase can be used. In a preferred non-limiting embodiment, the human glyoxylate reductase comprises the amino acid sequence of SEQ ID NO: 8. Therefore, a nucleic acid comprising the polynucleotide sequence of SEQ ID NO: 7 can also be used in accordance with the present invention.

In other non-limiting embodiments, the homologues derived from Arabidopsis thaliana or other higher plant sources can be used in place of the above-mentioned E. coli polypeptides encoded by the glc operon. A preferred Arabidopsis thaliana homologue comprises the amino acid sequence of SEQ ID NO: 10 and is encoded by a nucleic acid comprising the polynucleotide sequence of SEQ ID NO: 9.

In one embodiment, the second protein is an enzyme catalyzing the formation of tartronic semialdehyde. The tartronic semialdehyde may be formed from two molecules of glyoxylate under release of inorganic CO₂. Glyoxylate carboligase from bacterial or algal source, for example, as well as functional homologues from other sources are suitable for this purpose.

A preferred polypeptide having the activity of a glyoxylate carboligase comprises the amino acid sequence of SEQ ID NO: 12. Therefore, a nucleic acid comprising the polynucleotide sequence of SEQ ID NO: 11 can be used in accordance with the present invention.

In one embodiment, the third protein is an enzyme catalyzing the formation of glycerate from tartronic semialdehyde. Suitable enzymes for this purpose include, for example, tartronic semialdehyde reductase from bacterial or algal source, for example, as well as functional homologues from other sources.

A preferred polypeptide having the activity of a tartronic semialdehyde reductase comprises the amino acid sequence of SEQ ID NO: 14. Therefore, a nucleic acid comprising the polynucleotide sequence of SEQ ID NO: 13 can be used in accordance with the present invention.

For the purpose of expressing the nucleic acids which encode the polypeptides having the enzymatic activities in accordance with the present invention in plant cells, any convenient regulatory sequence can be used. In a non-limiting embodiment, the regulatory sequence provides transcriptional and translational initiation as well as termination regions. The transcriptional initiation may be constitutive or inducible. In a non-limiting embodiment, the coding region is operably linked to such regulatory sequences. Suitable regulatory sequences are represented, for example, by the constitutive 35S promoter which may be used for dicotyledonous plants. For monocotyledonous plants the constitutive ubiquitin promoter, for example, can be used. In a specific non-limiting embodiment, the maize ubiquitin promoter (GenBank: gi19700915) is employed as the regulatory sequence. Examples for inducible promoters include, without limitation, the light inducible promoters of the small subunit of RUBISCO, such as for example, the tomato rbcS promoter (GenBank: gi22624), and the promoters of the “light harvesting complex binding protein (lhcb)”, such as for example, the tobacco lhcb promoter (GenBank: gi1890636).

A polypeptide having the enzymatic activities in accordance with the present invention may comprise an amino acid sequence that targets the polypeptide. In particular, non-limiting embodiments, the amino acid sequence targets the polypeptide to the chloroplast, the chloroplast membrane and/or the cytoplasm. Suitable targeting sequences are known to the person skilled in the art. In a preferred embodiment, the chloroplast transit peptide derived from the ribulose-1,5-bisphosphate carboxylase gene from Solanum tuberosum (GenBank: G68077, amino acids 1-58) is used for targeting the polypeptides according to the present invention to the chloroplasts.

In another non-limiting embodiment, the polypeptide is directly targeted to the chloroplast using transformation of the chloroplast genome. In a specific non-limiting embodiment, the chloroplast is transformed by particle bombardment of leaf sections and integration by homologous recombination. Suitable vectors and selection systems are known to the person skilled in the art. The coding sequences for the polypeptides may be transferred, for example, in individual vectors or in one construct. The individual open reading frames may be fused to one or several polycistronic RNAs with ribosome binding sites added in front of each individual open reading frame in order to allow independent translation.

Phosphoglycolate phosphatase and glycerate kinase as well as RUBISCO are abundant enzymes inside plant chloroplasts. Thus, in other embodiments, these enzymes do not have to be transferred to chloroplasts to enable function of the novel biochemical pathway.

Introduction of the pathway in accordance with the present invention allows the direct reintegration of CO₂ released during the reaction pathway into the photosynthetic reactions. Accordingly, inorganic carbon is not lost in the mitochondria as described for photorespiratory reactions. Therefore, in accordance with the present invention, the products of photorespiration suppress photorespiration by increasing the CO₂ concentration at the site of fixation.

Exemplary methods to confirm that the novel pathway is functional inside the plant chloroplast include: (1) Studying whether the genes are expressed and the proteins accumulate inside the chloroplast; (2) Studying whether the metabolic intermediates of the pathway accumulate in the plant cell; (3) Studying any changes in the photosynthetic properties of the transformant by measurement of the photosynthetic property, which preferably includes the determination of the CO₂ compensation point by gas exchange measurements; and (4) Studying growth, biomass production and yield of the plants under different growth conditions, preferably under non-favourable conditions for C₃-plants.

The present invention provides plant cells, plant tissues, and plants. The plant, plant tissue, or plant cell comprises one or more nucleic acids which encode polypeptides having the enzymatic activities of (i) glycolate oxidase or glycolate dehydrogenase, (ii) glyoxylate carboligase and (iii) tartronic semialdehyde reductase.

Preferred embodiments of the nucleic acids introduced into the plant cells, plant tissues or plants are mentioned above.

Because of the enhanced photosynthetic potential, the plants that are produced in accordance with the present invention achieve one or more of the following properties: elevated yield per dry weight, improved drought and heat resistance, enhanced nitrogen-use efficiency, and reduced requirements for fertilization. These measurements are determined in comparison to control plants (for example, plants in which a nucleic acid of the invention has not been introduced).

The present invention will be better understood by the following exemplary teachings. The examples set forth herein do not and are not intended to limit in any manner the present invention.

EXAMPLES Example 1

Amplification of Genes from E. coli involved in Glycolate Metabolism

The genes encoding tartronic semialdehyde reductase, glyoxylate carboligase, and the open reading frames encoding the D, E, and F polypeptides from the glycolate oxidase operon were amplified from the genome of E. coli using the PCR method. Sequences were derived from public databases. The oligonucleotides were complementary to the start and the end of the coding regions, respectively, as provided in the sequence listing herein.

Example 2

Amplification of a cDNA Encoding Human Glyoxylate Reductase

The cDNA encoding glyoxylate reductase was amplified from human liver mRNA using the “Reverse-Transcriptase-PCR-method”. Sequences were derived from scientific literature (Rumsby and Cregeen, 1999). The mRNA was reverse transcribed using oligo-dT oligonucleotides as a start and the PCR was performed with oligonucleotides complementary to the start and the end of the coding regions, respectively, as provided in the sequence listing herein.

Example 3

Amplification of a cDNA Encoding a Homologue to the E. coli glcD open Reading Frame from Arabidopsis thaliana

An open reading frame with homology on the protein level to the glcD open reading frame from the glc operon from E. coli was identified as a genomic sequence from Arabidopsis thaliana in a public database. RNA was isolated from young leaf tissue and the coding sequence was amplified using the “Reverse-Transcriptase-PCR-method”. The mRNA was reverse transcribed using oligo-dT oligonucleotides as a start and the PCR was performed with oligonucleotides complementary to the start and the end of the coding regions, respectively, as provided in the sequence listing herein.

Example 4

Activity Measurement of the His-Tagged Versions Overexpressed in E. coli

The coding sequences of the genes as described in Examples 1–3 were cloned into the vector pET22b(+) [Novagen, Darmstadt, Germany] as a N-terminal translational fusion to six histidine residues. Proteins were overexpressed in E. coli strain ER2566 [NEB, Beverly, Mass., USA] and in part purified on a Ni²⁺-chelate-affinity matrix [Qiagen, Hilden, Germany] following the manufacturer's instructions.

Activities of tartronic semialdehyde reductase and glyoxylate carboligase were measured in crude extracts derived from overexpressing and non-overexpressing strains by techniques as disclosed before (Kohn, 1968; Chang et al., 1993). The results indicate that the proteins sustain their enzymatic function in translational fusion to six histidine residues and that the amplified sequences encode the respective enzymatic activities.

The activity of the Arabidopsis homologue to the E. coli glc operon (SEQ ID NO: 10) was measured in crude extracts from overexpressing and non-overexpressing bacterial strains as disclosed before (Lord, 1972). The results indicate that the Arabidopsis homologue is a true glycolate dehydrogenase capable of oxidising glycolate in the presence of organic cofactors. A diagram showing the relative activities is shown in FIG. 9.

Example 5

Construction of Expression Vectors for Plants

For plant transformation, the gene constructs were inserted into the binary plant expression vector pTRAKc, a derivative of pPAM (GenBank: AY027531). The expression cassette was flanked by the scaffold attachment region of the tobacco RB7 gene (GenBank: U67919). The nptII cassette of pPCV002 (Koncz and Schell, 1986) was used for selection of transgenic plants. Expression of genes is under constitutive control of the transcriptionally enhanced CaMV 35S promoter (Reichel et al., 1996). Proteins were expressed as a translational fusion to a chloroplast transit peptide derived from the ribulose-1,5-bisphosphate carboxylase gene from Solanum tuberosum (Gen Bank: G68077, amino acids 1–58). A schematic representation of the T-DNA is given in FIG. 6. Single constructs for tartronic semialdehyde reductase (TSR) and glyoxylate carboligase (GCL) were cloned into the MluI and Ecl136II restriction sites of the vector. A construct containing both genes was made by cutting a 4629 bp fragment from the construct containing the TSR coding sequence with restriction enzymes AscI and Scal and ligating it into the vector containing the GCL coding sequence cut with restriction enzymes PmeI and Scal. Before ligation, the protruding ends of the AscI restriction site were blunted with Klenow polymerase. A schematic representation of the final vector construct is given in FIG. 7. The vector is named pTRAc-rbCS1-cTP-TSR/GCL.

Example 6

Introduction of the Expression Vectors into Plants

The vector pTRAc-rbCS1-cTP-TSR/GCL was introduced into Agrobacterium tumefaciens strain GV3101 supplied by Dr. Koncz, Max-Planck-Institute for Breeding Research, Cologne, Germany via electroporation. Leaf sections of tobacco were transformed following techniques as described in (Schell et al., 1985). Calli were selected for resistance to Kanamycin and plants were regenerated from resistant calli. Potential transgenic lines were selected for the presence of the transgenes integrated into the genome using the PCR method. Accumulation of the transcripts and proteins, respectively, was shown using RT-PCR or Western analyses with antibodies to the C-terminal six-histidine fusion and highly expressing lines were selected.

Example 7

Plastidic Transformation of Tobacco

A vector was constructed allowing the simultaneous expression of all polypeptides necessary for the establishment of the proposed pathway by transformation of the chloroplast genome of tobacco. The part of the glc operon from E. coliencoding the glcD, glcE, and glcF polypeptides was amplified using the PCR method. The coding sequences for tartronic semialdehyde reductase (TSR) and glyoxylate carboligase (GCL) from E. coli were amplified and shine dalgarno sequences were added upstream of the coding sequence using the PCR method. A C-terminal translational fusion to six histidines was added to the GCL coding sequence. In each case oligonucleotides homologous to the beginning and the end of the respective coding sequences with extensions for the addition of the mentioned sequence elements were used. The complete construct was cut with NcoI and Bpu11021I and transferred into a vector for plastidic transformation that was cut with NcoI and XbaI (Bock, 2001). Before ligation, the protruding ends of the XbaI and Bpu1102I restriction sites were blunted with Klenow polymerase. A schematic representation of the vector insert is given in FIG. 8.

Chloroplasts of Nicotiana tabacum cv. Petit Havanna plants were transformed using particle bombardment. Transformed lines were selected using spectinomycin and streptomycin antibiotics as described before (Bock, 2001).

The present invention is not to be limited in scope by the specific embodiments described above. Many modifications of the present invention, in addition to those specifically recited above would be apparent to the skilled artisan using the teachings of the instant disclosure. Such modifications are intended to fall within the scope of the appended claims. All publications, patents and patent publications, cited in the instant specification are herein incorporated by reference in their entireties.

LITERATURE

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1. A method for producing C₃ plants with suppressed photorespiration and improved CO₂ fixation, the method comprising introducing into a C₃ plant cell, a C₃ plant tissue or a C₃ plant one or more nucleic acid sequences, each incorporated into nuclear genomic or chloroplastic DNA by transformation, wherein said one or more nucleic acid sequences encode polypeptides having the enzymatic activities of (i) an enzyme selected from the group consisting of glycolate oxidase and glycolate dehydrogenase, (ii) glyoxylate carboligase and (iii) tartronic semialdehyde reductase, wherein said introduction results in de novo expression of polypeptides having the enzymatic activities of (i) an enzyme selected from the group consisting of glycolate oxidase and glycolate dehydrogenase, (ii) glyoxylate carboligase and (iii) tartronic semialdehyde reductase and wherein said polypeptides are localized in chloroplasts of the plant produced.
 2. The method of claim 1 wherein each of said polypeptides comprises an amino acid sequence, that targets the polypeptide to the chloroplast.
 3. The method of claim 1 wherein said polypeptides having the enzymatic activity of a glycolate oxidase are derived from the E. coli glc operon.
 4. The method of claim 3 wherein said polypeptides comprise the amino acid sequences of SEQ ID NOS:2, 4 and
 6. 5. The method of claim 1 wherein said one or more nucleic acid sequences comprise the polynucleotide sequences of SEQ ID NOS:1, 3 and
 5. 6. The method of claim 1 wherein the polypeptide having the enzymatic activity of a glycolate dehydrogenase is a human glyoxylate reductase.
 7. The method of claim 6 wherein said polypeptide comprises the amino acid sequence of SEQ ID NO:8.
 8. The method of claim 1 wherein said one or more nucleic acid sequences comprise the polynucleotide sequence of SEQ ID NO:7.
 9. The method of claim 1 wherein the polypeptide having the enzymatic activity of a glyoxylate carboligase is derived from E. coli.
 10. The method of claim 9 wherein said polypeptide comprises the amino acid sequence of SEQ ID NO:
 12. 11. The method of claim 1 wherein said one or more nucleic acid sequences comprise the polynucleotide sequence of SEQ ID NO:
 11. 12. The method of claim 1 wherein the polypeptide having the enzymatic activity of a tartronic semialdehyde reductase is derived from E. coli.
 13. The method of claim 12 wherein said polypeptide comprises the amino acid sequence of SEQ ID NO:
 14. 14. The method of claim 1 wherein said one or more nucleic acid sequences comprise the polynucleotide sequence of SEQ ID NO:
 13. 15. An isolated C₃ plant cell, C₃ plant tissue or C₃ plant comprising one or more transgenic nucleic acid sequences encoding polypeptides having the enzymatic activities of (i) an enzyme selected from the group consisting of glycolate oxidase and glycolate dehydrogenase, (ii) glyoxylate carboligase and (iii) tartronic semialdehyde reductase.
 16. The plant cell, plant tissue or plant of claim 15 wherein each of said polypeptides comprises an amino acid sequence, and wherein each of said amino acid sequence targets said polypeptides to the chloroplast.
 17. The plant cell, plant tissue or plant of claim 15 wherein the polypeptide having the enzymatic activity of a glycolate oxidase is derived from the E. coli glc operon.
 18. The plant cell, plant tissue or plant of claim 17 wherein said polypeptides comprise the amino acid sequences of SEQ ID NOS:2, 4 and
 6. 19. The plant cell, plant tissue or plant of claim 15 wherein said one or more nucleic acid sequences comprise the polynucleotide sequences of SEQ ID NOS:1, 3 and
 5. 20. The plant cell, plant tissue or plant of claim 15 wherein the polypeptide having the enzymatic activity of a glycolate dehydrogenase is a human glyoxylate reductase.
 21. The plant cell, plant tissue or plant of claim 20 wherein said polypeptide comprises the amino acid sequence of SEQ ID NO:8.
 22. The plant cell, plant tissue or plant of claim 15 wherein said one or more nucleic acid sequences comprise the polynucleotide sequence of SEQ ID NO:7.
 23. The plant cell, plant tissue or plant of claim 15 wherein the polypeptide having the enzymatic activity of a glyoxylate carboligase is derived from E. coli.
 24. The plant cell, plant tissue or plant of claim 23 wherein said polypeptide comprises the amino acid sequence of SEQ ID NO:
 12. 25. The plant cell, plant tissue or plant of claim 15 wherein said one or more nucleic acid sequences comprise the polynucleotide sequence of SEQ ID NO:
 11. 26. The plant cell, plant tissue or plant of claim 15 wherein the polypeptide having the enzymatic activity of a tartronic semialdehyde reductase is derived from E. coli.
 27. The plant cell, plant tissue or plant of claim 26 wherein said polypeptide comprises the amino acid sequence of SEQ ID NO:
 14. 28. The plant cell, plant tissue or plant of claim 16 wherein said one or more nucleic acid sequences comprise the polynucleotide sequence of SEQ ID NO:
 13. 29. A C₃ plant produced by the method of claim
 1. 